1. Field of the Invention
The present invention relates, in general, to a driver circuit for generating current to an inductive load, and, more particularly, to a circuit used to drive a write coil of a read/write head for a hard disk drive.
2. Relevant Background
The computer marketplace continues to demand higher capacity and faster performance from data storage device such as hard disks and tape drives. Because of increased application file sizes, advanced operating systems, and multimedia applications, demand for hard disk drive capacity, for example, is doubling every year. This trend has pushed entry level drive capacities above several gigabyte (GB) levels. Technologies for storing and retrieving data from magnetic media must also be cost effective. Because lower cost per megabyte (MB) is also desired, the prior practice of simply adding more disks and heads to a hard drive is less and less effective.
Disk drives store binary encoded information as regions of magnetic flux on a media having a magnetic surface coating. It is desirable that these magnetic regions be encoded on the disk as densely as practical, so that a maximum amount of information may be stored. Disk and tape drive suppliers continue to increase areal densities, or the number of data bits per square inch, to meet the increasing demand for storage at competitive pricing. However, increasing areal density requires the write mechanism to produce smaller recorded patterns on the disk. Write head design and write driver design are key technologies needed to achieve these capacity increases.
To compensate for the weaker signals caused by smaller regions of magnetic flux for each byte, read heads are designed to fly only a few microinches from the magnetic media. Because this distance is already much less than the size of a dust particle, it is unlikely that further improvements can be achieved by moving the heads closer to the media. Moreover, reliability becomes a significant concern as the heads are moved closer to the media.
The magnetic regions are created by passing current through a coil of a magnetic write head. Binary data can be encoded by switching the polarity of the current through the write coil.
The current in the write coil is provided by a write head driver circuit and must be carefully controlled. The data rate (i.e., the rate at which bits can be written onto the media) is determined largely by the rate at which the current can be switched in the write head driver circuit. It is desirable to have a write head drive circuit that quickly switches current to the desired polarity and magnitude to support high disk rotation speeds with small magnetic regions. Also, the driver circuit must raise the current amplitude to a level sufficient to ensure the flux generated by the write coil is adequate to saturate the magnetic media while limiting the current below levels that will result in xe2x80x9cbloomingxe2x80x9d of the written magnetic region into adjacent regions of the media.
Due to the inductive nature of a write circuit head and the parasitic capacitance(s) and resistance(s) associated with the write circuitry, ringing effects occur in the write current signal which tends to delay the settling of the write current to its final DC value. These ringing effects are seen as overshoot and undershoot. Overshoot can be tolerated to a large extent, but in the extreme will result in writing data to adjacent regions. Undershoot is usually undesirable as the undershoot may result in writing a bit of the opposite polarity than is intended at either the target location or an adjacent location. Hence, ringing can adversely affect precise placement of the magnetic region on the magnetic media and areal density.
In conventional driver circuits, ringing is worsened by higher slew rates. Higher slew rates increase the magnitude of the ringing and make it more difficult to control. One option when ringing effects are present is to simply wait for the write current to settle to a steady state value before enabling the next transition for encoding a bit. However, this decreases the areal the density of bit encoding by the write circuit and so is undesirable.
Other approaches to control ringing use active circuits to generate cancellation currents that limit overshoot and undershoot. This approach maintains acceptable encoding density, but increases circuit complexity and cost. Also, delays associated with the active circuitry tend to limit the effectiveness of this approach and slow the slew rate.
A conventional write driver circuit comprises an H-bridge configuration using four switches. In an H-bridge circuit, one leg of the bridge is always trying to drive current into the inductive load. In other words, the H-bridge is always coupling the power supply voltage onto one of the inductor nodes and ground to the other inductor node by appropriately activating the bridge switches. Although the switching devices that make up an H-bridge circuit can be made very fast, so long as the H-bridge is trying to drive current into the inductor, the slew rate of current through the inductor is limited by inductor physics.
The slew rate is governed by a fundamental equation describing current when voltage is applied to an inductor:       V    =          L      ⁢              xe2x80x83            ⁢                        ⅆ          i                          ⅆ          t                      ,            which      ⁢              xe2x80x83            ⁢      can      ⁢              xe2x80x83            ⁢      be      ⁢              xe2x80x83            ⁢      rewritten      ⁢              xe2x80x83            ⁢      as      ⁢              :            ⁢              xe2x80x83            ⁢              V        L              =                  ⅆ        i                    ⅆ        t            
Given a write head where the inductance (L) is predetermined, the only way to increase slew rate (di/dt) is to increase the voltage (V) applied across the inductor. However, in practical electronic systems the available voltage is limited to the supply voltages provided by the system (e.g., 5.0 V in a personal computer system). Moreover, increasing the voltage is often not possible due to the limitations of the semiconductor devices used to implement the write driver circuit.
Another limitation of H-bridge drivers is that parasitic device elements, namely parasitic capacitances associated with switching transistors, are coupled so as to oppose rapid current switching in the load. Charging the parasitic elements, which must occur before switching can occur, robs current from the load thereby lowering the di/dt from its theoretical maximum.
Hence, a need exists for a circuit for driving inductive loads, and particularly a circuit for driving write heads in a magnetic recording media that provides high slew rate with controlled ringing. Moreover, a need exists for a high slew rate driver circuit that does not require excessive voltages or additional voltage supplies, and that can be implemented using simple, low cost circuitry.
Briefly stated, the present invention involves an inductive load driver circuit including a first switch that switches between a conductive state and a non-conductive state selectively applies a first power supply potential to a first side of the inductive load in response to a control signal. A second switch that switches between a non-conductive state and a conductive state selectively applies a second power supply potential to a second side of the inductive load in response to the control signal. The control signal places a control node of the second switch at a lower potential than the second side of the inductive load while the second switch is in the conductive state. In operation, a steady state current in a first direction is driven through the inductive load. The nodes of the inductive load are placed in a high impedance state, after which a steady state current is driven in a second direction through the inductive load.